A final crushing defeat (8)
I believe the answer is:
waterloo
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">a final crushing defeat</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for waterloo that I've seen before include "Total or crushing defeat" , "Decisive battle" , "Ultimate defeat - London station" , "Napoleon's final battle" , "battle in Belgium" .)